[xen staging-4.11] tools/ocaml: libxb: Avoid to use String_val() when value is bytes

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commit e8067088d255333487e01bfa59f4a0524172719c
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Mon Mar 30 18:50:08 2020 +0100
Commit:     Ian Jackson <iwj@xxxxxxxxxxxxxx>
CommitDate: Fri Mar 19 13:46:25 2021 +0000

    tools/ocaml: libxb: Avoid to use String_val() when value is bytes
    
    Commit ec7d54dd1a "ocaml/libs/xb: Use bytes in place of strings for
    mutable buffers" switch mutable buffers from string to bytes. However
    the C code were still using String_Val() to access them.
    
    While the underlying structure is the same between string and bytes, a
    string is meant to be immutable. OCaml 4.06.1 and later will enforce it.
    Therefore, it will not be possible to build the OCaml libs when using
    -safe-string. This is because String_val() will return a const value.
    
    To avoid plain cast in the code, the code is now switched to use
    Bytes_val(). As the macro is not defined in older OCaml version, we need
    to provide a stub.
    
    Take the opportunity to switch to const the buffer in
    ml_interface_write() as it should not be modified.
    
    Reported-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Acked-by: Christian Lindig <christian.lindig@xxxxxxxxxx>
    (cherry picked from commit 78686437e949a85a207ae1a0d637efe2d3778bbe)
---
 tools/ocaml/libs/xb/xs_ring_stubs.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/ocaml/libs/xb/xs_ring_stubs.c 
b/tools/ocaml/libs/xb/xs_ring_stubs.c
index 473787064a..7537a23949 100644
--- a/tools/ocaml/libs/xb/xs_ring_stubs.c
+++ b/tools/ocaml/libs/xb/xs_ring_stubs.c
@@ -36,6 +36,14 @@
 
 #define GET_C_STRUCT(a) ((struct mmap_interface *) a)
 
+/*
+ * Bytes_val has been introduced by Ocaml 4.06.1. So define our own version
+ * if needed.
+ */
+#ifndef Bytes_val
+#define Bytes_val(x) ((unsigned char *) Bp_val(x))
+#endif
+
 CAMLprim value ml_interface_read(value ml_interface,
                                  value ml_buffer,
                                  value ml_len)
@@ -44,7 +52,7 @@ CAMLprim value ml_interface_read(value ml_interface,
        CAMLlocal1(ml_result);
 
        struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-       char *buffer = String_val(ml_buffer);
+       unsigned char *buffer = Bytes_val(ml_buffer);
        int len = Int_val(ml_len);
        int result;
 
@@ -103,7 +111,7 @@ CAMLprim value ml_interface_write(value ml_interface,
        CAMLlocal1(ml_result);
 
        struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-       char *buffer = String_val(ml_buffer);
+       const unsigned char *buffer = Bytes_val(ml_buffer);
        int len = Int_val(ml_len);
        int result;
 
--
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